1. Field of Invention
The present invention relates to a wavelength-converted semiconductor light emitting device on a chip-scale package.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes such as surface-emitting lasers (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
III-nitride light emitting devices often emit blue or UV light. To form an LED that emits white light, one or more wavelength converting materials such as phosphors are often disposed in the path of the blue or UV light emitted from the LED. For example, for an LED that emits blue light, a single phosphor that emits yellow light may be used, or two phosphors that emit green and red light may be used. Some of the light emitted by the LED is wavelength-converted by the phosphor. The wavelength-converted light emitted by the phosphor mixes with unconverted light emitted by the LED such that the overall appearance of light from the device is white.
FIG. 1 illustrates a process for forming a wavelength-converted light emitting device mounted on a chip-scale package, described in more detail in US 2010/0279437. A chip-scale package refers to a package for the light emitting device that is attached to the semiconductor light emitting device structure in a wafer-scale process. In process 102 of FIG. 1, LEDs are formed on a growth wafer. In process 104, a carrier wafer is temporarily bonded to the device wafer. A removable adhesive is first applied over the top of the device wafer then a carrier wafer is bonded to the top of the device wafer. In process 106, the device wafer is flipped over and the growth wafer is removed. In process 108, the n-type layer exposed by removing the growth wafer is roughened to improve light extraction. In process 110, a window wafer is bonded to the device wafer. The window wafer provides mechanical strength to the device wafer for subsequent processing. The window wafer may include a wavelength converting structure for modifying the emission spectrum to provide a desired color such as amber for signal lights or multiple colors for a white light emitter. The structure may be a ceramic phosphor, a suitable transparent substrate or carrier such as a sapphire or glass layer, or a filter such as a distributed Bragg reflector. In process 112, the carrier wafer is removed from the device wafer. In process 114, the device wafer is mounted from the bottom side to a stretch film. The stretch film may be a blue tape, a white tape, a UV tape, or other suitable materials that allows adhesion to a flexible (expandable) substrate. In process 116, the LEDs in the device wafer are singulated into individual dice, for example using a laser, a scribe, or a saw. The LED dice may have edge emission that degrades color-over-angle uniformity. In process 118, the stretch film is expanded to laterally separate the LED dice to create the spaces between neighboring dice. In process 120, a reflective coating is applied over the tops of the LEDs and in the spaces between them. In process 122, the reflective coating in the spaces between the LED dice is optionally broken or weakened (e.g., cleaved). In process 124, the stretch film is expanded again to further laterally separate the LED dice. In process 126, portions of the reflective coating on the top of the LED dice is removed. Afterwards only portions of the reflective coating on the sides of the LED dice remains. Portions of the reflective coating on the sides of the LED dice may control edge emission, improve color-over-angle uniformity, and improve brightness. In process 128, the LEDs are flipped over and transferred to another stretch film to expose n-type bond pads and p-type bond pads on the LED dice for testing in process 130.